Changes in ecosystem aboveground and soil carbon stocks through 25+ year remeasurement of post-harvest second-growth and old-growth stands in dry coastal Douglas-fir forests.

Abstract:

Information on changes in aboveground and especially soil C during post-clearcut succession in forests over 100 years is very limited and often based on a space for time substitution, ie. comparisons of different aged stands of similar type and disturbance history, a chronosequence. Such information is needed to verify forest C budget estimates and to develop C offset projects. In 1991 the Coastal Forest Chronosequence project was initiated on Vancouver Island, BC to examine how forests recover in second-growth stands and compare to old forests. It includes four sites in dry east-island Douglas-fir forests, each site with a post-clearcut chronosequence of three stands (Regeneration 7-9yr, Immature 25-45yr, Mature 65-95yr) plus an Old-growth control stand (>250yr) (16 stands x 3 plots/stand= 48 plots). Total ecosystem C stocks including aboveground tree and dead wood as well as soil (forest floor and mineral soil to 55cm) were measured in 1992. Aboveground C stocks were remeasured in 2005-06, while aboveground and soil C were remeasured in 2016-19. Repeated measurements allowed for comparisons of individual stand trajectories to polynominal regression curves fit to C values for mean stand age at each measurement. In 1992, live biomass C increased with age but was still lower in M vs O stands. Dead wood C tended to be lowest in I stands compared to all other stand ages which were similar. Soil C did not significantly differ with stand age. In 2019, overall trends in live biomass C for polynominal curves, ( 0-3MgC/ha R~ 10yr, 25-75 MgC/ha I ~ 50yr, 100-200 MgC/ha M ~100yr, a maxima of 200-250 MgC/ha at 250yrs, and 150-200 MgC/ha O ~320yrs) were similar across the 3 measurements. Variation in stemwood biomass occurred among sites within a stand age especially M stands, though slopes for individual stands were similar to the overall trend. Stemwood decline in two stands in 2016-19 were due effects of adjacent logging on 2 plots. The overall trend was for biomass to decline in O stands, however the slope of individual O stands was positive, with growth increasing over the last two measurements. Similar trends were observed when values of live plus dead wood biomass were examined. Decreases in total woody biomass did not occur in two stands as losses in live biomass was accounted for in dead wood. Changes in soil C over the 25yr period will be discussed.